A physical cell identifier (PCID) is an identification of a sector or cell at the physical layer. The Primary Sync Signal (PSS) or physical layer ID and the Secondary Sync Signal (SSS), which is a physical layer cell ID group number, determine the PCID. In some networks, the physical layer ID can have values between zero and two, while the cell ID group number can have values between 0 and 167. The PCID can be determined from the physical layer ID and the physical layer cell ID group number as three times the cell ID group number plus the physical layer ID number. Thus, the PCID can take values between 0 and 503.
Downlink reference signal is a signal that exists at the physical layer. The downlink reference signal delivers a reference point for downlink power. When a user equipment (UE) attempts to determine downlink power (e.g., the power of the signal from a base station), the UE measures the power of the downlink reference signal. Downlink reference signals are carried by multiples of specific Resource Elements (RE). The downlink reference signal RE position and downlink reference signal sequence is based on the physical layer ID.
Because the number of available PCIDs are limited (e.g., in existing systems PCIDs can only have values between 0 and 503), PCID reuse is required in a cellular network. PCIDs are typically assigned manually (e.g., by an operator) when a sector is brought into service. However, when neighboring sectors use the same PCID, a collision can occur when user equipment (UE) attempts to connect to one of the sectors. Similarly, even if there is not a PCID collision, neighboring sectors may use the same physical layer ID and thus the same positions for transmitting their downlink reference signals. As a result, downlink reference signals of neighboring sectors may have time-frequency collision.